Motor endplate remodeling in some cases with congenital

Anna Fidziańska, Zofia Glinka

Folia Neuropathol 2010; 48 (3): 200-205

AbstractThe architecture of motor endplates in three cases with congenital myasthenic syndrome (CMS) was compared withultrastructure of the normal control neuromuscular junction (NMJ). The remodeling of postsynaptic region wasobserved in all three individuals. The most conspicuous abnormalities seen in the slow channel syndrome was thevacuolization and disorganization of secondary synaptic clefts which extended for beyoned the border of NMJ.Degenerated postsynaptic nuclei and junctional sarcoplasm were an additional feature of presented syndromes.The quite different feature of NMJ was observed in the DOK-7 deficient syndrome. The appearance of small, pale ter-minal axons, poorly developed postsynaptic membrane with the sparse secondary synaptic clefts and degeneratedpostsynaptic nuclei suggested impairment of postsynaptic region maturation. The conjunction of postsynaptic mem-brane paucity and its degeneration was a specific structural feature observed in the third syndrome with no estab-lished genetic defects.

Introduction cated proteins or their loss affects the architecture

and function of NMJ. Using electron microscopy, we The motor endplate is highly specialized in mor- analysed endplate architecture and described ultra-phology and molecular composition connection structural abnormalities that occur in the endplates ofbetween nerve and muscle cell which is able to trans- patients with congenital myasthenic syndromes.duce signals from the nerve terminal to skeletal mus-cle fibre. The development and differentiation of the Material and methodsneuromuscular junction (NMJ) is a multisteps processrequiring coordinated interaction between muscle cell The structure of motor endplates was analysed inand nerve terminal. The initial step in NMJ formation three patients with disorder neuromuscular transmis-and differentiation require activity of numerous cyto- sion. The first patient was a young woman with theplasmic proteins [11]. Genetic defects in presynaptic, slow channel syndrome and a mutation in AChRsynaptic and postsynaptic junction are known as con- epsilon subunit [9]. The second patient was a boygenital myasthenic syndromes (CMS) [2,6-8,13]. Trun- with mutation in the DOK-7 gene (Engel et al. data not

Fig. 1. A) Control motor endplate (EP) with

published) who had a similarly affected younger

brother. The third patient was a man with undeter- *mined genetic diagnosis (a mutation in the AChRgene was excluded) who had clinical and electrophy-siological signs of CMS. The quadriceps femoris mus-cle of all patients was analysed. As a control motorendplates of unaffected quadriceps femoris muscleswere investigated. For electron microscopy specimens were fixed inthe 1% glutaraldehyde in phosphate buffer and post-fixed in 1% osmium tetroxide in the same buffer. Then Fig. 2. Case 1. The significantly widened primarythey were dehydrated and embedded in Spurr resin. synaptic cleft (asterisk) with dense punctuateThin sections double stained with uranyl acetate and granules. × 45 000.lead citrate were examined with JEM electron micro-scope. space of 5.0 nm, called primary synaptic cleft, sepa- rates the terminal nerve and muscle cell plasmaResults membrane. Plasma membrane of the muscle cell is The normal synapse contains three major struc- lined with basal lamina which contains numeroustural elements: the presynaptic nerve terminal that is proteins regulating synapse formation and activity.capped by a Schwann cell, the primary synaptic cleft The secondary synaptic clefts in normal EP are welloccupied by basal lamina and secondary synaptic developed. They are numerous, long, closely packedclefts formed by folds in the postsynaptic membrane and often branched (Fig. 1B). In the case with muta-(Fig. 1A). The hallmark of the nerve terminal is the tion in AChR epsilon subunit, the shape and size ofappearance of the synaptic vesicles which are located the terminal nerve varied as did the extent of foldingprecisely across synaptic cytoplasm. The synaptic of the postsynaptic membranes. A few swelling mito-

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Fig. 4. Case 1. Degenerated postsynaptic nucleus.

× 22 000.

chondria were observed in endplates. The synaptic

space was significantly widened with decreasing den-sity of basal lamina as well as with the presence ofnumerous dense punctuate granules (Fig. 2). The ab-normality of the postsynaptic area of NMJ was thedestruction of postjunctional folds and the appear-ance of membrane-bounded vacuoles (Fig. 3A,B). Thejunctional sarcoplasm was filled with vacuole-likestructures ranging in size from 0.05 to 1 µm. They are Fig. 5. Case 2. A small endplate (EP) with swell-empty or filled with degenerated membranes. Postsy- ing mitochondria and widened primary cleft.naptic nuclei showed different stages of degeneration × 22 000.(Fig. 4) including vacuolization, heterochromatin con-densation. The appearance of various inclusions in normal NMJ. Terminal axons showed a small size,nuclear matrix was frequently observed. swelling mitochondria a decreased number of synap- In the case with a mutation in the DOK 7 gene, the tic vesicles (Fig. 5). The primary synaptic defts weremotor enplate revealed abnormality of pre- and post- widened. The number of postsynaptic folds (secon-synaptic regions compared to the architecture of dary synaptic clefts) was significally reduced. The post-

202 Folia Neuropathologica 2010; 48/3

Motor endplate remodeling in some cases with congenital myasthenic syndrome

Fig. 7. Case 3. Extremely widened primary synap-

tic cleft. × 36 000.

Fig. 6. Case 2. The significantly reduced postsy-

naptic folds. Vacuolated junctional sarcoplasm. × 18 000.

synaptic membrane was straight, with few short fold-

ings (Fig. 6). The junctional sarcoplasm showed dege- Fig. 8. Case 3. The paucity of secondary postsy-nerated mitochondria, membrane-bound structures naptic clefts and widened primary synaptic cleft.(Fig. 6) and degenerating synaptic nuclei. × 45 000. In the third case, electron microscopy analysisrevealed small nerve terminals, extremely widened observed in all affected cases. In contrast to thesesynaptic space (Fig. 7) reduced junctional folds (Fig. 8) marked changes in the postsynaptic region, notableand a very large destroyed postjunctional region. lack of degeneration of the nerve terminals was seen.Postjunctional sarcoplasm created with tubular net- Degeneration of postsynaptic junctions and their losswork showed numerous myelin structures and degen- were observed in the case with mutation in epsilonerated nuclei (Fig. 9A,B). unit of AChR gene as well as in the genetically unde- termined case. The postsynaptic folds reorganization,Discussion destruction and vacuolization of sarcoplasmic compo- In this report, we describe motor endplate remo- nents have been observed in both cases, although indeling in three cases with impaired neuromuscular the case with a mutation in epsilon AChR gene accu-transmission diagnosed as the congenital myasthenic mulation of vacuolar structures was the dominatingsyndrome (CMS). Ultrastructural analysis showed feature. In addition, vacuolated mitochondria, myelinthat in all three cases, the postjunctional region of structures and autophagic vacuoles were frequentlyNMJ underwent reorganization and deformation. Sig- found.nificant widening of the synaptic space with the Postsynaptic nuclei exhibited varied stages ofappearance of dense, punctuate granules was degeneration in both investigated cases. Such reor-